1. Field of the Invention
The present invention relates to a polygon mirror drive motor for use in a laser beam printer, a photocopier or the like, and is equipped with a polygon mirror for deflecting and scanning a laser beam.
2. Description of the Related Art
In a laser beam printer or a photocopier is utilized a polygon mirror drive motor. In the polygon mirror drive motor, a polygon mirror configured by combining a plurality of mirror facets into a polygonal shape, seen in plain view, is mounted in a rotor portion that rotates with respect to a stator. With this, the polygon mirror can rotate at high speed to deflect and scan a laser beam being incident upon the plurality of the mirror facets. A scanning light beam produced by deflecting and scanning the laser beam with the polygon mirror creates an image on a photoreceptor or paper.
By the way, since a polygon mirror that is produced with a plurality of mirror facets formed into a shape of a polygon is deformed by a centrifugal force when the polygon mirror rotates at high speed, there exits a light scanning optical device that is capable of correcting the deformation (Refer to Japanese Patent Application Publication H09-43529).
In addition, there is a polygon mirror that realizes a high degree of optical precision in a motor mounting hole, a motor mounting base and the like, while reducing its weight and costs by arranging four mirror facets in square in plain view.
FIG. 1 illustrates a top view of an example of a polygon mirror used in a light scanning optical apparatus of related art. FIGS. 2A and 2B illustrate a top and a cross-sectional view of another example of a polygon mirror of related art, respectively.
The example of a polygon mirror 100 used in the related art light scanning optical apparatus illustrated in FIG. 1 is disclosed in Japanese Patent Application Laid-open Publication H09-43529. The polygon mirror 100 has a deflective reflection surface 101a that has been shaped into a slight concave when the polygon mirror 100 stays stationary, as shown in a dashed line in FIG. 1. When the polygon mirror 100 rotates at high speed around a rotation axis 101b, the deflective reflection surface 101a deforms so as to bulge out to take a shape of a deflective reflection surface 101a′ that is shown in a full-line in FIG. 1 due to a centrifugal force caused by high-speed rotation. Therefore, while a focal point of the polygon mirror 100 lies in front of a surface to be scanned when the mirror 100 stays stationary, the focal point falls upon the surface to be scanned when the mirror 100 rotates at high speed, thereby compensating a positional change of the focal point stemming from the deformation caused by the rotation of the polygon mirror 100.
Another example of a rotation polygon mirror is disclosed in Japanese Patent Application Laid-open Publication H09-61743 as shown in FIGS. 2A and 2B. The rotation polygon mirror 200 is shaped into a hexahedron having six square faces of 4 mm thick. The rotation polygon mirror 200 has a center core 201 in the center thereof and an optical face core 202 that surrounds the center core 201. Through the center core 201 is pierced a hole 201a for a motor shaft and formed of polycarbonate containing glass fiber having a rigidity. The optical face core 202 is formed of polycarbonate as thermoplastic material having optical properties suitable for a rotational polygon mirror. With the above configuration, the above publication recites that the mirror 200 can be produced with high positional accuracy in a motor shaft hole, a base level or the like and thereby with high optical accuracy while keeping production costs for the mirror 200 reduced.
In case of the polygon mirror 100 of an example used for the conventional optical scanning optical apparatus shown in FIG. 1, it is necessary to design the deflective reflection surface 101a so that the surface 100a takes a shape of an appropriate concave, taking account of an amount of the deformation caused when the deflective reflection surface 101a is forced to bulge out by the centrifugal force at the time of high-speed rotation. However, it is difficult to estimate in advance an amount of the deformation of the deflective reflection surface 101a, and also difficult to create the deflective reflection surface 101a into a concave in accordance with the estimation. On top of that, considering a technology trend in that a further increase in the rotation speed of the polygon mirror 100 has been expected in view of a recent progress in a laser beam printer or a photocopier in terms of highly definitive and precise imaging, a deformation of deflective reflection surface 101a caused by a centrifugal force has now drawn a great deal of attention, since the centrifugal force at the time of high-speed rotation of the polygon mirror 100 is proportional to the number of rotations to the 2nd power. For this reason, it is still more difficult to estimate the amount of the deformation of the deflective reflection surface 101a. 
Moreover, an upper surface, a lower surface, and four mirror facets are easily obtained with high dimensional precision, the four mirror facets being configured to surround the upper and the lower surface and to become a side face of the polygon mirror, since the polygon mirror 200 shown in FIGS. 2A and 2B is formed into a hexahedron. However, since the outside of a main core 201 having a high rigidity is covered with an optical surface core 202 having a favorable optical property, a production of the rotation multifaceted mirror (polygon mirror) 200 needs an increased number of processes, thereby resulting in high production costs.
In view of the above, it has been desired that a polygon mirror drive motor which can control the deformation of each mirror facet caused by the centrifugal force of high-speed rotation is obtained while producing easily a polygon mirror having sufficient dimensional precision by connecting a total of four mirror facets into a square in plain view.